1. Field of the Invention
The present invention relates to a mobile communication system with a service area covered by a plurality of cells in which a communication of a mobile station is realized by sequentially switching the cells in communication with the mobile station according to the movement of the mobile station.
2. Description of the Background Art
In a mobile communication system having a service area covered by a plurality of cells with base stations provided therein, in order for a mobile station which is moving through the service area to make a continuous communication, it is necessary to carry out the handover procedure for sequentially switching the base stations in communication with the mobile station according to the movement of the mobile station.
Conventionally, this handover procedure has been carried out as illustrated by the sequence chart of FIG. 1 for an exemplary case in which a mobile station MS is initially in communication with a base station BS1 through a channel ch1 (S0). When the mobile station MS detects the cell transition from a cell of the base station BS1 to a cell of another base station BS2 (S1), the mobile station MS transmits a cell transition notice as a handover activation signal to the base station BS1 (S2). In response, the base station BS1 recognizes that a handover to the base station BS2 is required, and transmits a channel set up signal to the base station BS2 (S3). Then, the base station BS2 selects a new channel ch2 (S4), and starts the signal transmission through the channel ch2 (S5).
Next, the base station BS2 transmits an activation completion signal for indicating that the channel ch2 has been activated to the base station BS1 (S6). In response, the base station BS1 transmits a channel specification signal for specifying the channel ch2 to the mobile station MS (S7). Then, the mobile station MS switches its communication channel from the channel ch1 to the channel ch2 (S8). Subsequently, the mobile station MS starts the signal transmission through the channel ch2 to the base station BS2 such that a reception synchronization for this new communication channel ch2 is established at the base station BS2 (S9) and the mobile station MS and the base station BS2 are set in communication with each other (S10).
After that, the base station BS2 transmits a channel release command signal to the base station BS1 (S11). In response, the base station BS1 stops the signal transmission through the channel ch1 (S12) and releases the on-line channel with respect to an exchange station (S13). Then, the handover procedure is completed as the base station BS1 transmits a channel release completion signal to the base station BS2 (S14).
In this conventional handover procedure, at a time of switching from the channel ch1 to the channel ch2, if this mobile communication system uses the frequency division multiple access (FDMA) scheme, a frequency must be changed by switching a synthesizer, and consequently the communication of the mobile station MS will be instantaneously disconnected during a switching time.
Thus, in the conventional mobile communication system in which the mobile station is capable of transmitting and receiving signals through only one channel at a time, there has been a problem that it is associated with a temporal lowering of the service quality due to the occurrence of the interruption of a speech flow or data dropping at a time of switching the base stations in the handover procedure.